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Why UV?

Spectrum of light ranges from the infrared at wavelengths longer than visible light to the ultraviolet at wavelengths shorter than visible light. Ultraviolet rays have shorter wavelengths than visible light. A wavelength, the distance between the crests of two waves, is often measured in units called nanometers. A nanometer (nm) is a billionth of a meter, or about 1/25,000,000 inch. Wavelengths of visible light range from about 400 to 700 nm. Ultraviolet wavelengths range from about 1 to 400 nm and are beyond the range of visible light.

Ultraviolet rays with wavelengths shorter than 300 nm are extremely effective in killing microorganisms. The most effective sterilizing range for UV is within the C bandwidth (UVC). This range is called the germicidal bandwidth. UVC has been used in hospitals for decades to sterilize surgical instruments, water, and the air in operating rooms. Many food and drug companies use germicidal lamps to disinfect various types of products and their containers.

UV disinfects through a photochemical process. The contaminants that pollute the indoor environment are almost entirely based upon organic or carbon-based compounds. These compounds breakdown when exposed to high intensity UV at 240 to 280 nm. Short-wave ultraviolet light can destroy DNA in living microorganisms and breakdown organic material found in indoor air. UVC’s effectiveness is directly related to intensity and exposure time.

UV rays must strike the contaminants directly in order to penetrate the microorganism and breakdown its molecular bonds. This bond breakage translates into cellular or genetic damage with the germs rendered harmless by robbing them of the ability to reproduce.

About Ultraviolet (UV) Light Used for Air Disinfection

What is UV or ultraviolet light?

Ultraviolet light is part of the spectrum of electromagnetic energy generated by the sun. The full spectrum includes, in order increasing energy, radio waves, infrared, visible light, ultraviolet, x-rays, gamma rays and cosmic rays. Since UV is not visible, it is technically not “light”, but use of the term “ultraviolet light” is so widespread that, it will be used here. Most sources of light generate some UV. For air disinfection, UV is generated by electric lamps that resemble ordinary fluorescent lamps.

What is germicidal UV?

This is UV of a specific type (253.7nm wavelength) known to kill airborne germs that transmit some infections from person to person within buildings. Germicidal UV is aimed at the upper room air so that only airborne microbes are directly exposed. Room occupants are exposed only to low levels of reflected UV – levels below that known to cause eye irritation. Germicidal UV has been used safely and effectively in hospitals, clinics and laboratories for more than 60 years. UV does not prevent transmission of infections (e.g. colds) by direct person to person contact.

Is UV harmful?

We are all exposed to the UV in sunlight. UV exposure can be very harmful, or harmless, depending on the type of UV, the type of exposure, the duration of exposure, and individual differences in response to UV. There are three types of UV:

UV-A – Long wave UV, also known as “black light”, the major type of UV in sunlight, responsible for skin tanning, generally not harmful, used in medicine to treat certain skin disorders.

UV-B – A small, but dangerous part of sunlight. Most solar UV-B is absorbed by the diminishing atmospheric ozone layer. Prolonged exposure is responsible for some type of skin cancer, skin aging, and cataracts (clouding of the lens of the eye).

UV-C – Also known as “short wave” UV, includes germicidal (253.7nm wavelength) UV used for air and water disinfection. Unintentional overexposure causes transient redness and eye irritation, but does NOT cause skin cancer or cataracts.

How UV Light Works

Germicidal ultraviolet (UVC) light kills cells by damaging their DNA. The light initiates a reaction between two molecules of thymine, one of the bases that make up DNA. The resulting thymine dimer is very stable, but repair of this kind of DNA damage–usually by excising or removing the two bases and filling in the gaps with new nucleotides–is fairly efficient. Even so, it breaks down when the damage is extensive.

The longer the exposure to UVC light, the more thymine dimers are formed in the DNA and the greater the risk of an incorrect repair or a “missed” dimer. If cellular processes are disrupted because of an incorrect repair or remaining damage, the cell cannot carry out its normal functions. If the damage is extensive and widespread, the cell will die.

UV Dosage Table

Please note that many variables (flow rate, distance of microorganism to the UV light, irradiation time) take place in a real world environment that make actual calculating of the UV dosage very difficult. However, it is proven that UV light will kill any DNA-based microorganism given enough UV dosage. UV breaks down DNA on a cumulative basis.

The following are incident energies of germicidal ultraviolet radiation at 253.7 nanometers necessary to inhibit colony formation in microorganisms (90%) and for 2 log reduction (99%):